A known height position sensing device for use with a control circuit for controlling the height position of a blade in the case of a motor grader exclusively consists of a mechanical height-position sensor for determining, by means of monitoring, the height relative to a height reference defined by a tight rope extending along the surface to be graded by the motor grader. In the case of this known height-position sensing device, the rope is monitored by a feeler tube or by a feeler arm whose angular position, which depends on the relative height with respect to the rope, is detected by a transformer and converter into a signal indicative of the height. The monitoring of the rope can only provide a reference height for a first strip which is parallel to the rope and which is graded by the motor grader in a first working cycle. When additional strips are produced, control of the height position of the blade must be effected with reference to the configuration of the just finished first strip extending on the ground. For this purpose, a feeler ski equipped with a further mechanical height-position sensor is provided, said feeler ski sliding on the just finished first strip so as to form a height reference when the next strip is being produced. This mechanical monitoring of the ground by means of a feeler ski which is supported such that it is vertically movable is complicated and sensitive by nature. In particular, the mechanical feeler ski limits the possible operating speed when the motor grader is in operation. Further limitations result from the fact that problems may arise due to the feeler ski when the motor grader is turning round or moving in the reverse direction.
It is true that, the field of construction machines, it is known to use also contactless systems for the purpose of distance measurement, said systems including in particular ultrasonic measuring systems, but, at least on the basis of the presently available technology, an ultrasonic distance measuring system is not suitable for reliably detecting the position of a reference rope so that such systems have hitherto not been used for controlling the height-position of the blade in the case of a motor grader.